Warwickshire was a discrete coalfield that ceased mining in 2013 due to a catastrophic fire at the last mine, Daw Mill Colliery which had to be sealed at the surface allowing the fire to eventually burn itself out. With the end of deep mining a traditional source of employment came to an end that had existed in the area for several hundred years leaving only the prospect of coal winning from the occasional open caste operations that might have planning permission granted as part of a ground restoration project at the northern or eastern fringes where the coal seams outcrop at or close to the surface.
The Warwickshire coalfield was, as suggested a basin that outcropped at the northern, western (where it was ever present) and eastern boundaries and sank down towards the south where the coal dipped below the Warwickshire sandstones. To the western, Birmingham edge the coal was missing in any workable quantity and was replaced by sandstone formations. As the coal dropped down towards the south the coal seams thickened and were believed to be in continuity with the Burford coal basin below Oxfordshire. The mining plan was to follow the coal towards the south and with the sinking of future working, pumping or ventilation shafts to take the coal to that end goal at considerable depth. Originally this was to be achieved by removing coal from not just Daw Mill but also via Coventry Colliery which operated towards the south-eastern edge of the deep coalfield, closed in 1991.
The Warwickshire coalfield is unlike many of the simpler lateral geological coalfields in that it is in part heavily faulted and the coal seams are at steep angles leading to large unconformities both due to the strata incline and faulting within the underground workings formed as the coal was chased deeper. The history of the mine development is important to the understanding of the hydrological situation beneath the county post deep mining and to the possible risks and scenarios that will need to be understood now pumping to dewater the coal faces has ceased.
There have been several bland statements that there is no risk of pollution or at least not for many years expected at the surface. Such statements being a way of giving rise to a sense of false security. Years pass swiftly and those who had knowledge of the workings are lost, thus leaving a knowledge vacuum. The coalfield generally has been amalgamated into the landscape, spoil heaps or bings reprofiled into the patterns of the surrounding land: only the colliery villages giving a hint of the voids that lie below the surface of the ground.
The main and present concern with regard to the Warwickshire Coalfield is the threat to the Warwickshire Aquifer that overlies it. Since the closure of Daw Mill Colliery the underground workings have started to fill with water, both from the overlying aquifer and via the surface deposition of rainfall at the outcrop then passing through the interlinked workings of past coal workings up dip.
The Water Framework Directive identifies the Warwickshire Aquifer as a Principal Aquifer and is in part a protected water source by the Environment Agency. The whole area has historically been an important resource for public water supply. Prior to the Elan Reservoir supply being opened in 1910 the Warwickshire aquifer was the major source of drinking water for Birmingham. The potable supply for much of the north of the county including Atherstone and Nuneaton.
The issues of minewater are common at the surface in many parts of the UK: usually evidenced by a red, ochre stained river bed and a lack of aquatic life. In the case of Warwickshire the rivers are unlikely to suffer this fate: before such a time the aquifer would be infiltrated leaving much of this major resource damaged and possibly beyond remediation in the short or medium term. Such a threat, it is hoped will not come about due to the state’s responsibility being fielded by Coal Authority who are tasked as the ‘nongovernmental’ agency to protect the environment from such events.
The key event that triggered such hydrological concerns for the Warwickshire Coalfield was the closure of Daw Mill: the abrupt end to mining due to a gob fire that had started spontaneously. As no one had been injured or was trapped below ground the event allowed for a rapid sealing of the mine workings. The intension being to starve the subterranean conflagration of oxygen and limit the risk of the coal face beginning to combust, similarly the smaller seams up dip of the workings, which might have led to issues of subsidence and air pollution from smoke and fumes reaching the outside environment.
Coal close to the surface when opencast at the opencast operation that removed much of the evidence of the mine top at Griff Colliery.
Daw Mill Colliery was originally sunk as the ventilation shaft for the workings operated from collieries to the north. It was later developed as, first, a twin shaft pit and later a drift was developed so as to allow continuous extraction from the working faces to surface rather having to transfer tubs to a shaft and winding to surface. The original ventilation shaft was put in place to allow both Kingsbury and Dexter pits to extend their coal take. As the coal winning progressed the second shaft was sunk allowing the two original mines to be closed and investment to be concentrated at Daw Mill. The mine needed to pump minewater drainage so as not to flood the coalface down dip. This water was pumped to surface where the most conspicuous pollutant, iron, was removed in settlement lagoons and the treated flow left the site by a dedicated pipeline to be discharged to the River Tame. The local watercourse (River Bourne) that bordered the colliery site, being unable to assimilate either the volume or other contaminants within the drainage. The discharge to the River Tame was possible due to the volumes of treated sewage emanating, just upstream, from the Minworth Sewage Treatment Works being capable of diluting contaminants from the mine water: both flows being considered to have a beneficial effect to each other providing further oxygenation to the River Tame in the mixing of the waters. Both Minworth and Daw Mill discharges further enabled the water take from the River Tame by the Hams Hall power stations complex (now closed) that needed to abstract large volumes of cooling water to allow the largest power complex in Europe to function.
The post mining hydrological environment of the North Warwickshire Coal field is complex. There are many uncertainties as to some linkages with unrecorded mine workings. The map above shows operational mines from the nationalisation of the coal industry in 1946/7 but there were many small coal winning ventures from the 17th Century that are linked in to the recorded workings.
The majority of the overlying aquifer is recognised as a secondry water supply source but both potable supply and bottled mineral water are sourced close above worked coal seams.
The above plan details the river/stream flows across and around the main part of the Warwickshire Coalfield with little in the way of flow from the central area. This is a positive element as the rivers are above the main worked coal seams and the majority of coalmine derived water will be held within the workings.
Water abstractions from across the coalfield.
Much of the coalfield is protected as a groundwater protection zone which is also covered by protection from farming practices as a Nitrate Sensitive Zone. The area to the south east of the protected area are in part a protected major aquifer.
Daw Mill Colliery site is shown as on the cusp of a secondary, important aquifer that borders a lesser secondary aquifer. The issues of aquifer infiltration are a concern for the areas both to the north (towards Kingsbury via the Dexter workings) and to the south of the Daw Mill workings which are the first workings to be inundated with the secondary contamination from the fire able to infiltrate the fire damaged geology (severely fractured?) below ground.
The threats to the aquifers in Warwickshire are not admitted to by government or the agencies of the government. The unofficial stance is at present that little is known as it is a new issue and that more research is needed to be carried out.
The most pressing needs are:
- A study of abandonment plans for those collieries and pits closing post 1874 (the date of legal need to lodge such plans with the state).
- Research the pumping information for mines primarily linked to Daw Mill. These being Kingsbury and Dexter.
- Minewater monitoring shaft data and borehole logs held by the Coal Authority to be made available if there are any.
- A monitoring regime to be set up if there is a lack of current information.
The closure of Daw Mill by fire rapidly brought about an event that was destined to happen in a few years due to the country’s energy policy if not the world economics of coal. The sudden nature of the event though shielded the other concerns with regard to hydrogeological concerns. Now the fog of the closure has abated there is a pressing need to understand the full implications of the closure of coal mining in this area of England.
Coal Shearer working at Daw Mill 2009. (© UK Coal/Daily Mail)
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Coal Authority www.gov.uk/government/organisations/the-coal-authority
Environment Agency www.environmentagency.gov.uk
Northern Mine Society www.nms.org.uk
Clean Rivers Trust www.cleanriverstrust.co.uk
Thanks to Warwickshire County Council Records Office.